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Natural sciences

Grade 9

Energy, forces and machines

Module 22


Performance (effort/work) is important, but the SPEED at which it is performed is also important.


Explaining the concept power and calculating it

[lo 2.3; lo 2.4]


We measure this in J/s, or Watt (W)

1J per second provides 1 Watt of power

  • The term Watt is derived from the name of James Watt, who was responsible for improvements to the steam engine.
  • The electricity in your home is measured in kilowatt-hour (kWh) .
  • One kilowatt-hour is the energy output of a 1 kW appliance in one hour.
  • The longer such an appliance is operated, the more energy is used. A 2 kW heater that is operated for three hours will therefore use 6 kW energy.
  • Complete: …… kW per hour x …… hour = 6 kW.
  • The cost of a kWh varies but comes to approximately R0,35 in the Western Cape.


Measure your own power capacity

  • Identify a stairway in the school and measure the height of a step.
  • Multiply this by the number of steps.
  • Determine your weight.
  • Run up the stairs and take your time in seconds.

Complete the following table:

Aspect Measurement
Step height
Work performed: F x s


1. What was your power?

2. Explain the following:

a) Kilowatt

b) Kilowatt-hour

3. Find out what the power rating of the average car (your educator's car, for instance) is.

4. Would you be prepared to push your educator's car all the way home? Why not?

5. Why would it be easier for the car engine to do this, even though you might have enough time and help from your friends?

6. Calculate the cost of operating a 3 000 W heater for a month (July) for 4 hours per day at a tariff of R0,35 per kWh.


Were you able to apply knowledge and do the calculations?

[LO 2.3; LO 2.4]


Doing research on power and performing relevant calculations

[lo 1.1; lo 1.2; lo 2.2; lo 2.4]

Find out what 1 kWh of electricity costs in your municipal area.

Suppose you use the following appliances during the day:

  • Computer for 2 hours
  • Television for 3 hours
  • Lamp for 4 hours
  • Kettle for 15 minutes
  • Stove for 45 minutes
  • Hairdryer for 15 minutes
  • Determine the average power of each of these items by reading the specifications given on the appliances. Complete the table.
Electrical appliance Power (kW)

Now complete the following cost assessment:

APPLIANCE POWER (kW) as above TIME Usage(hours) Energy Usage (kW/h) Cost @ R1,50 per hour


1. What is your deduction?

2. Are you aware of the need to use energy more economically? Give your own opinion of this.

3. Calculate the following:

a) A mountaineer whose mass is 75 kg climbs a 320 m rock face in 20 minutes. Calculate his power.

b) A large cupboard weighing 150 kg has to be carried up a stairway approximately 5 m high during a move.

  • How much work is performed?
  • What is the performance rating of the workers if two men are able to do this in 10 minutes?

Assessment of RESEARCH on appliances

Were you able to obtain all the required information and do the calculations?

Could you categorise information?

[LO 1.1; LO 1.2; LO 2.2; LO 2.4]


LO 1: Scientific investigations:

The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, technological and environmental contexts.

This is evident when the learner:

  • plans investigations;
  • conducts investigations and collects data.

LO 2: Constructing Science Knowledge:

The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

This is evident when the learner:

  • categorises information;
  • interprets information;

2.4 applies knowledge.





Power = Labour (work) Time

Labour = Power (F) × s

Weight = Mass × 10 =  Kg

= F × s Time =

2. a) 1000 Watt, in other words, 1 000 × Joule/second

b) The amount of kW that is delivered or consumed in one hour

3. Larger cars approximately 100 kW; smaller cars approximately 70 kW

4. –

5. Engine has greater power / capacity for doing work

6. First calculate total consumption of energy (kWh):

= kW × h (hour)= 3 kW × 124 h= 372 Kw

Then calculate the costs:

= 372 × R0,35= R130,20


The power per hour of appliances (more of less):

  • Pocket calculator 0,0006W
  • Microwave oven 750W
  • Drill 500W
  • Radio 28W
  • Toaster 800W
  • Fan 100W
  • Iron 1 200W
  • Stove 1000W
  • Computer 250W


1. The greater the power, the higher the cost of using it

2. Debate

3. a) W = F x s= (75 x 10) x 320= 750 x 320= 240 000 J

Power = 240 000 J ( 20 × 60 ) = 240 000 1 200 = 200 W ( or 0,2 lW ) alignl { stack { size 12{ ital "Power"= { { size 11{"240"`"000"`J}} over { size 11{ \( "20" times "60" \) }} } } {} #size 11{ {}= { { size 11{"240"`"000"}} over { size 11{1`"200"}} } } {} # size 11{ {}="200"`W` \( ital "or"`0,2 ital "lW" \) } {}} } {}

b) W = F x s = (150 x 10) x 5= 7 500 J

Drywing = Arbeid Tyd = 7 500 J 600 s = 12 , 5 W alignl { stack { size 12{ size 11{ ital "Drywing"= { { size 11{ ital "Arbeid"}} over { size 11{ ital "Tyd"}} } }} {} #size 11{ {}= { { size 11{7`"500"`J}} over { size 11{"600"`s}} } } {} # size 11{ {}="12",5`W} {}} } {}

 One worker's performance = 12,5  2= 6,25 W

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Natural sciences grade 9. OpenStax CNX. Sep 15, 2009 Download for free at http://cnx.org/content/col11069/1.1
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